1. Field of the Invention
The present invention relates to a multiplex transmission system that demultiplexes the following data into plural channels again after the multiplex transmission system multiplexes and transmits data received from plural channels, particularly relates to a system that multiplexes packets and others in an SDH/SONET frame and others by time division and transmits the frame and its equipment.
2. Description of the Related Art
For a multiplex transmission system that multiplexes packets received from plural channels and transmits them on a transmission line, there is a multiplex transmission system that multiplexes and transmits packets according to a packet multiplexing system. Generally, as the length of a packet is variable, the multiplex transmission system temporarily stores a packet received from each channel in a buffer, multiplexes it after the processing of a header and transmits it on a transmission line.
There is also a multiplex transmission system that multiplexes packets by time division and transmits them. Such a time-division multiplex transmission system multiplexes a packet received from each channel by time division and transmits it on a transmission line. In case data from plural channels are multiplexed in units of byte and are transmitted, a frame is generally generated and the data of each channel is multiplexed in order from a reference position of the frame.
In a multiplex transmission system according to a packet multiplexing system of these, to execute the processing of a packet the length of which is variable, a receive buffer that can temporarily store a packet having the maximum length is required every line. To control multiplexing, further large capacity is generally required for a transmission buffer. The transmission buffer means a buffer for storing a packet to be transmitted on a transmission line and the receive buffer means a buffer for storing a packet received from the transmission line.
In the meantime, in the multiplex transmission system according to the packet multiplexing system, as it is awaited to read a packet from a buffer to multiplex packets from plural channels on one transmission line, a delay of a packet fluctuates.
In the multiplex transmission system according to the packet multiplexing system, when packets from plural channels are multiplexed on one transmission line, blocking may occur.
As described above, though the multiplex transmission system according to the packet multiplexing system has an advantage that packets from plural channels different in a transfer rate can be effectively multiplexed, it also has problems that a buffer having large capacity is required, a delay fluctuates and blocking occurs.
Therefore, in case such problems are required to be avoided, the multiplex transmission system according to time-division multiplexing is preferable. Various interfaces different in a transfer rate and others can be housed in each channel by using a generic framing procedure (GFP) standardized based upon T1X1.5 for example for an interface in the multiplex transmission system according to time-division multiplexing.
The multiplex transmission system according to time-division multiplexing requires a frame on a transmission line, however, for a frame, there are two types of a frame defined on one's own terms and a standardized frame.
In a multiplex transmission system, a communication device may be required to be provided between a transmitter and a receiver of the multiplex transmission system because the distance of transmission is long.
In case a frame is defined on one's own terms, there is a problem that the existing communication device and a general-purpose communication device cannot be used between the transmitter and the receiver.
Therefore, in this type of multiplex transmission system, it is desirable that a worldwide standardized interface is used. For a worldwide standardized interface of a transmission line, there is a synchronous digital hierarchy/synchronous optical network (SDH/SONET). For a standardized interface such as SDH/SONET, a frame format is predetermined. In case a multiplex transmission system is defined as a system for multiplexing data of each channel in an SDH/SONET frame, a general-purpose communication device according to SDH/SONET can be used between a transmitter and a receiver.
In case data from plural channels is multiplexed in an SDH/SONET frame in units of byte for example by time division, it is required that a fragment of a payload is not used or data of each channel is multiplexed asynchronously with the SDH/SONET frame unless the number of channels is the number of channels in which data from the channels can be exactly multiplexed in a payload of one SDH/SONET frame.
In case data of each channel is multiplexed asynchronously with an SDH/SONET frame, a position in which data of each channel is multiplexed cannot be known based upon information shown in the form of an overhead and others of the SDH/SONET frame on the receive side.
In case a fragment of a payload is not used, a part of the payload becomes useless.
Recently, each chip vender provides chip set LSI that terminates an SDH/SONET frame, however, of them, there is chip set LSI which merely synchronizes in units of byte and does not output a signal showing a position in an SDH/SONET frame. In case such chip set LSI is used, a position of a multiplexed channel cannot be known on the receive side even if data of each channel is exactly multiplexed.
To enable the detection of a position of each channel multiplexed asynchronously with an SDH/SONET frame, a frame has only to be further generated every multiplexed channel in the SDH/SONET frame and fields for a synchronizing signal and a channel identifying signal have only to be provided in each frame. However, then, a band of each channel is reduced by quantity in which the fields for the synchronizing signal and the channel identifying signal are provided.